Coated Metal Powders for Laser Powder Bed Fusion (L-PBF) Processing: A Review

Post-Processing Effect on the Corrosion Resistance of Super Duplex Stainless Steel Produced by Laser Powder Bed Fusion

This study examines the microstructural characteristics and corrosion resistance of super duplex stainless steel (SDSS) produced through laser powder bed fusion (LPBF). The analysis shows that the as-printed samples mainly exhibit a ferritic microstructure, which is due to the fast-cooling rates of the LPBF technique. X-ray and microstructure analyses reveal the presence of minor austenite phases in the ferritic matrix. The process of solution annealing led to a more balanced microstructure. Analyses of corrosion resistance, such as potentiodynamic polarization tests and EIS, indicate that heat treatment has a significant impact on the corrosion behavior of SDSS. Solution annealing and stress relieving at 400 °C for 1 h can improve corrosion resistance by increasing polarization resistance and favorable EIS parameters. However, stress relieving at 550 °C for 5 h may reduce the material's corrosion resistance due to the formation of chromium nitride. Therefore, stress relieving at 400 °C for 1 h is a practical method to significantly enhance the corrosion resistance of LPBF-printed SDSS. This method offers a balance between microstructural integrity and material performance.

The Preparation of an Ultrafine Copper Powder by the Hydrogen Reduction of an Ultrafine Copper Oxide Powder and Reduction Kinetics

Ultrafine copper powders were prepared by the air-jet milling of copper oxide (CuO) powders and a subsequent hydrogen (H2) reduction. After milling, the particle size and grain size of CuO powders decreased, while the specific surface area and structural microstrain increased, thereby improving the reaction activity. In a pure H2 atmosphere, the process of CuO reduction was conducted in one step, and followed a pseudo-first-order kinetics model. The smaller CuO powders after milling exhibited higher reduction rates and lower activation energies compared with those without milling. Based on the unreacted shrinking core model, the reduction of CuO powders via H2 was controlled by the interface reaction at the early stage, whereas the latter was limited by the diffusion of H2 through the solid product layer. Additionally, the scanning electron microscopy (SEM) indicated that copper powders after H2 reduction presented a spherical-like shape, and the sintering and agglomeration between particles occurred after 300 °C, which led to a moderate increase in particle size. The preparing parameters (at 400 °C for 180 min) were preferred to obtain ultrafine copper powders with an average particle size in the range of 5.43-6.72 μm and an oxygen content of less than 0.2 wt.%.

Simulation of 316L Stainless Steel Produced the Laser Powder Bed Fusion Process

Additive manufacturing is increasingly being used in the production of parts of simple as well as complex shapes designed for various areas of industry. Prevention of errors in the production process is currently enabled using simulation tools that have the function of predicting possible errors and, at the same time, providing a set of information about the behaviour of the material in the metal additive manufacturing process. This paper discusses the simulation processes of 316L stainless steel produced using the laser powder bed fusion (L-PBF) process. Simulation of the printing process in the Simufact Additive simulation program made it possible to predict possible deformations and errors that could occur in the process of producing test samples. After analysing the final distortion already with compensation, the simulation values of maximum deviation -0.01 mm and minimum -0.13 mm were achieved.

Effects of Mo Particles Addition on the Microstructure and Properties of 316 L Stainless Steels Fabricated by Laser Powder Bed Fusion

Application of the 316 L stainless steel (SS) is limited by its relatively low wear resistance, insufficient strength, and poor corrosion resistance in special environments. To this end, effects of Mo particles addition on the microstructure, mechanical properties, and corrosion resistance of the laser powder bed fusion (LPBF) 316 L SS are investigated in this study. The results show that the addition of Mo particles from 0 wt.% to 10 wt.% can modify the crystal orientation and improve the strength, wear resistance, and corrosion resistance of LPBF 316 L SSs. Particularly, the LPBF 316 L SS forms a biphasic structure with a similar ratio of α-Fe to γ-Fe with 5 wt.% Mo addition. As a result, the corresponding samples possess both the excellent toughness of austenitic SSs and the high strength and corrosion resistance of ferrite SSs, which reaches a high tensile strength of about 830 MPa, together with a low friction coefficient of 0.421 μ. Since the Mo particles addition is beneficial to increase the content of Cr₂O₃ on the samples surface from 13.48% to 22.68%, the corrosion current density of 316 L SS decreases by two orders of magnitude from 569 nA to 6 nA, while the mechanical properties remain favorable. This study is expected to serve as a reference for the preparation of LPBF SSs with excellent integrated performance.

Investigation of the Properties of 316L Stainless Steel after AM and Heat Treatment

Additive manufacturing, including laser powder bed fusion, offers possibilities for the production of materials with properties comparable to conventional technologies. The main aim of this paper is to describe the specific microstructure of 316L stainless steel prepared using additive manufacturing. The as-built state and the material after heat treatment (solution annealing at 1050 °C and 60 min soaking time, followed by artificial aging at 700 °C and 3000 min soaking time) were analyzed. A static tensile test at ambient temperature, 77 K, and 8 K was performed to evaluate the mechanical properties. The characteristics of the specific microstructure were examined using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The stainless steel 316L prepared using laser powder bed fusion consisted of a hierarchical austenitic microstructure, with a grain size of 25 µm as-built up to 35 µm after heat treatment. The grains predominantly contained fine 300-700 nm subgrains with a cellular structure. It was concluded that after the selected heat treatment there was a significant reduction in dislocations. An increase in precipitates was observed after heat treatment, from the original amount of approximately 20 nm to 150 nm.

Molecular Dynamics Simulation of Fe-Based Metal Powder Oxidation during Laser Powder Bed Fusion

Because the laser powder bed fusion process is generally completed in a confined space and in a very short time, it is difficult to study material oxidation during this process using traditional methods. To address this knowledge gap, in this work, we used molecular dynamics (MDs) based on a reaction force field (ReaxFF) to clarify the atomic-level interaction mechanism between metal atoms and oxygen molecules during laser powder bed fusion. The ReaxFF potential energy model has variable charges that can dynamically handle charge changes between atoms and the breaking and formation of chemical bonds that occur during oxidation reactions. We investigated the effects of laser power, scanning speed, region position, and oxygen concentration on powder oxidation. The results show that the laser power and scanning speed affected the oxidation degree by changing the energy input density, and the oxidation degree increased with the energy input density. Different forms of oxidation occurred near the melt channel due to the existence of a temperature gradient, and the degree of oxidation increased with the temperature. Atoms in the metal powder model underwent selective oxidation, which was related to the potential energy of their atomic position. A larger potential energy made it easier for iron atoms to overcome the energy barrier during the initial stage of oxidation, making them easier to oxidize.